In PET scanners pixellated scintillators are typically used to convert incident radiation particles to bursts of photons with a wavelength in the UV or visible spectrum. The scintillators usually comprise a matrix of scintillator elements with a base area in the order of 1×1 mm2 to 4×4 mm2. The scintillation events are detected by photosensors coupled to the scintillator elements. State of the art PET scanners use solid-state photosensors, e.g. silicon photomultipliers (SiPMs), typically comprising an array of single photon avalanche diodes (SPADs) configured to break down responsive to impingement of a photon.
Alternatively, monolithic scintillators may be used which consist of a large block of scintillator material. Monolithic scintillators are typically coupled to an array of photosensors configured to localize scintillation events at different scintillator element locations within the monolithic scintillator.
The size of the scintillator element locations that can be identified is a primary factor determining the spatial resolution of the resulting image. Thus, small scintillator element locations are desired to increase the resolution. In the quest for higher resolution solid-state nuclear imaging systems, Anger logic has been used to attain a resolution which is superior to the size of a single photosensor. By coupling the scintillator and the photosensors with a light guide that spreads the emitted scintillation light onto several photosensors and identifying the scintillator element locations with Anger logic, resolution can be improved. Since Anger logic relies on information from neighboring photosensors to identify the scintillator element location, Anger logic becomes inaccurate when information of some of the photosensors is missing, e.g. at gaps and edges of the photosensor array.
Patent application US2014/175294A1 discloses a photon detector that includes a detector array with single photon avalanche diode, SPAD, detectors configured to break down responsive to impingement of a photon. Trigger circuitry is configured to generate a trigger signal responsive to break down of a SPAD detector of the SPAD detector array. Time to digital converter circuitry may generate a digital time stamp for the trigger signal.